Atraumatic Toe Pain: Acute Calcific Periarthritis.

Acute calcific periarthritis (ACP) is a self-limiting but often underrecognized condition that can closely mimics more serious joint pathologies such as septic arthritis. We present the case of a 43-year-old woman who developed acute pain and swelling of the right second toe after a 4-mile run. Initial evaluation in the emergency department (ED) included point-of-care ultrasound and C-arm imaging, which revealed periarticular calcification at the distal interphalangeal joint. However, the diagnosis of ACP was not made at that time. The patient was treated with nonsteroidal anti-inflammatory medication and referred for outpatient orthopedic evaluation. Despite symptomatic improvement and partial resolution of the calcification on radiographs one week later, the diagnosis remained unclear until two months later, when an orthopedic oncologist identified the condition as ACP. ACP results from hydroxyapatite crystal deposition in periarticular tissues and is most commonly seen in the shoulder but may also involve the hands, feet, and other joints. It presents with acute monoarticular pain, swelling, erythema, and limited range of motion. These features may overlap with crystal arthropathies, infection, and intra-articular injury. Radiographs and ultrasound are useful tools for identifying characteristic calcifications and ruling out alternative etiologies. Recognition of ACP in the ED is important to avoid unnecessary interventions, consultations, and diagnostic delays. This case highlights the clinical and imaging features of ACP and emphasizes the importance of including it in the differential diagnosis of acute monoarticular joint pain.